The broad, long-term goal of this project is to advance our understanding of the functional origin of optical signals used in breast tissue characterization and imaging. This will be accomplished using Broadband Diffuse Optical Spectroscopy (DOS), a non-invasive technique that combines multi-frequency frequency-domain photon migration (FDPM) with time-independent near-infrared (NIR) spectroscopy to quantitatively measure bulk tissue absorption and scattering spectra. Our approach has two major components: 1) advancing a hand-held DOS reflectance probe for clinical measurements and 2) combining DOS with MRI and exogenous molecular contrast agents for pre-clinical animal model studies. Each technology development activity is accompanied by basic science and translational research. Translational studies are designed to test the feasibility of optical methods in evaluating breast disease risk, distinguishing between malignant and benign lesions, and characterizing the impact of therapies (e.g. hormone replacement therapy (HRT) and neoadjuvant chemotherapy). Broadband DOS instruments will be placed in 3 clinical performance sites: 1) the Avon Breast Center in the Chao Family Comprehensive Cancer Center at UC Irvine, 2) the Carol Franc Buck Breast Center in the UC San Francisco Comprehensive Cancer Center, and 2) the UCI Beckman Laser Institute Medical Clinic (BLIMC). A translational research infrastructure will be developed in these sites that includes defining procedures for correlating DOS with conventional radiology methods, epidemiologic models and histopathology. Clinical studies will be completed in order to derive predictive "tissue optical indices" (TOI) for diagnosing disease, monitoring therapies, and understanding risk. Project 1 emphasizes determining how NIR spectral features are affected by alterations in breast physiology and biochemical composition. Our results will provide critical information for Projects 2 and 3 on the optimal utilization of wavelengths and modulation frequencies for breast tissue functional imaging. Extensive utilization of Instrumentation, Software/Computation, and Molecular Probes Cores will take place throughout the proposed research.